US Senate candidate Elizabeth Warren’s campaign said Monday that she raised $6.9 million in the first three months of the year, continuing her staggering pace and doubling the figure collected by incumbent Scott Brown.

Warren, a Democrat, is proving an especially prolific fund-raiser in her first run for public office, setting the pace nationally among Senate candidates in the final three months of 2011. A national Democratic official said the party expects Warren will again outrank all other Senate candidates, from both parties, in the recently completed fund-raising quarter, which ended March 31.

Official figures from all Senate campaigns will not be made public by the Federal Election Commission until later this month.

In any other Senate race in the country, Brown’s haul would also be eye-popping. He outraised every Senate candidate nationally, apart from Warren and two self-funded candidates, in the last three months of 2011.

His campaign said Friday that the Republican raised $3.4 million in the first quarter of this year, giving Brown a total of about $15 million in the bank.

Before Warren entered the race, some Democrats were concerned that the party would not be able to overcome Brown’s financial advantage. But Warren’s campaign said Monday that she has $11 million on hand. That’s about $4 million less than Brown, who had money left over from his 2010 special election victory.

That gap may evaporate quickly, given that Warren has roughly doubled Brown’s fund-raising since August, when she opened her exploratory committee.

The race will probably be the most expensive election, by far, in state history.

Campaign contributions are watched closely by political insiders and activists, who view them as an index of enthusiasm for a candidate and a measure of how active a campaign can be in building an organization and putting up ads as the election draws nearer.

Warren’s fund-raising prowess has helped her clear the primary field of all but one competitor, North Shore attorney Marisa DeFranco, who has not been able to mount as sophisticated a campaign operation.

Warren has excited the Democratic base around the country, with her fiery defense of liberal economic policy and appeal to the middle class.

Many Democrats are also motivated to win back the seat that was long held by Edward M. Kennedy, before Brown stunned the political establishment by defeating Attorney General Martha Coakley in the 2010 special election.

That passion has translated into thousands of online donations. A liberal political action committee known as the Progressive Change Campaign Committee said Monday that it has raised $678,803.64 for Warren so far.

Overall, 83 percent of her donations were $50 or less in the most recent quarter, her campaign said.

Despite Warren’s fund-raising advantage, Brown has shown in polls that he remains popular with voters and will be difficult to unseat. The most recent Globe poll showed the race was deadlocked, with Brown holding a 2-point lead, within the poll’s margin of error. Brown was also viewed as more likable than Warren by 57 percent of respondents.

Warren has raised the majority of her donations out of state, though her campaign says it believes she has still collected more money in Massachusetts this year than Brown.

“The incredible enthusiasm we have seen from people across the Commonwealth who are contributing to this campaign shows the strong grass-roots momentum behind Elizabeth’s fight for middle-class families,’’ Warren campaign manager Mindy Myers said in a statement Monday.

“Professor Warren’s fund-raising continues to be mostly out-of-state money from extremely liberal donors and special interests that are trying to influence the Massachusetts election,’’ spokesman Colin Reed said in an e-mail.

Neither side has released its full fund-raising report, lengthy documents showing names, occupations, and addresses of donors who give more than $200, along with the size of their donations. Candidates do not have to file that information until April 15.

It can take more than a week for the Federal Election Commission to put the reports online so that independent observers can calculate more specifically who is funding each campaign.

That information vacuum allows the campaigns to selectively release information in hope of winning the media war.

The Brown campaign released its fund-raising tally Friday, before the holiday weekend, probably anticipating that Warren would hold a financial advantage and would wait until this week to release her figures. That allowed the Brown campaign to publicize his total before it faced comparison.

Hoping to underscore Warren’s dependence on out-of-state donors, the Brown campaign also said Friday that about 71 percent of Brown’s first-quarter donors were Massachusetts residents. But the campaign would not say how much money that represented or detail the size of those donations.

The Warren campaign, in turn, attempted to rebut criticism that its money is coming from out of state. Last week, her campaign released its in-state-only fund-raising total, $2.5 million, and made the assertion that Warren raised more money within Massachusetts than Brown.

When Warren’s campaign released its tally Monday, it gave few additional details on where the money had come from.